The present invention relates to a method for determining the rotation speed of a drill bit, and, more particularly, to such a method intended to determine this speed in real time from data captured at one point of the string.
During the drilling of an oil well, the motor unit for the string of pipe, which motor unit is mounted on the surface, rotates at a constant speed of between 50 and 150 revolutions per minute. However, the friction produced between the drill bit and the bottom of the well, or between the pipes and the wall of the well, may cause the bit to slow down or indeed to stop periodically. As during this time, the motor unit continues to rotate at one end of the string of pipe, the latter tends to twist about its longitudinal axis until the force exerted is greater than the friction effect braking the bit. At that moment, the string of pipe relaxes and the bit starts to rotate again and can reach point rotation speeds of the order of 150 to 400 revolutions per minute. As wells often follow deviated trajectories, contact between the string and the wall of the well occurs quite frequently.
It is obvious that the behaviour of the bit has a significant effect on the progress of the drilling. Thus, it is desirable for the head driller to be alerted to periodic rotation-speed instabilities of the bit in order that he can modify the parameters of the drillingrotation speed of the motor unit, mud flow rate or weight applied to the tool--and thus to ensure optimal drilling.